1. Field of the Invention
The present invention relates to a light-emitting diode (LED), with its light-fetching efficiency exalted, to a method for the fabrication thereof and to an LED lamp using the LED.
2. Description of the Prior Art
A light-emitting device endowed with an improved efficiency of energy consumption (external quantum efficiency) has been yearned for with a view to promoting the saving on energy. In the GaN-type light-emitting diode superposed on a sapphire substrate, the external quantum efficiency of the conventional light-emitting diode (LED) operating in the neighborhood of 382 nm has been 24% according to JP-A 2002-164296. The external quantum efficiency, as the product of “(internal quantum efficiency)×(voltage efficiency)×(light fetching efficiency),” is decomposed into the three elements. The two elements other than the actually measurable voltage efficiency (about 90 to 95%) are incapable of actual measurement. Notwithstanding the levels of these elements remain yet to be clarified, the improvement of the internal quantum efficiency that has mainly resorted to the quality of crystal and the optimization of construction has been chiefly studied. On the other hand, as an example of the improvement achieved in the efficiency of fetching light, the method has been in vogue since a long time ago, which represses the total reflection on the interface between resin and air by coating an LED chip with a resin approximating a semiconductor in refractivity, thereby allowing the emitted light to penetrate the resin efficiently, and further forming the resin surface in a spherical shape. Then, as an example of the realization of the increase of the efficiency of fetching light to about twice the ordinary level by grinding a substrate in the shape of an inverse mesa, Cree Corp. of the U.S. has been marketing the product embodying this increase under the designation of X-Bright series.
As a way of implementing the measure to allay the trend of a semiconductor crystal toward dislocation, a method that consists in rendering the surface of a semiconductor crystal substrate irregular and then allowing a semiconductor layer to grow has become generally known. In the case of a semiconductor of the nitride of an element of Group In, for example, it has been demonstrated that the density of dislocation can be allayed by forming grooves in the pattern of stripes on the surface of a sapphire substrate and then inducing epitaxial growth thereon of a GaN buffer layer liable to grow at a low temperature and further thereon of the crystal of the semiconductor of the nitride of an element of Group In liable to grow at a high temperature. It is held that for this decrease of the density of dislocation, the grooves are preferably inclined by an angle of 60° or more. No mention, however, is made of the efficiency of fetching light (refer, for example, to JP-A 2002-164296 and K. Tadamoto, et at., Japanese Journal of Applied Physics, 2001, Vol. 40, p. L583–L585). However, these references do not touch upon the efficiency of fetching light
Generally, the light-emitting device (LED) has been incapable of causing a ray of light having an angle of incidence larger than the angle of total reflection to be fetched from a light-emitting layer to the exterior because the refractive index of the light-emitting layer is larger than the refractive index of the external medium.
Object of the present invention is to provide a light-emitting device improved in the efficiency of fetching light by introducing irregularities having a lateral face inclined to the interface between two layers differing in refractivity, thereby enabling the totally reflected ray of light to be fetched to the exterior, a manufacturing method thereof and a LED lamp using the light-emitting device.
To begin with, the circumstances that have brought the present invention to perfection will be described by way of simulation below.
With the object of estimating the efficiency of fetching light and the internal quantum efficiency that are actually unmeasurable, the present inventor estimated the efficiency of fetching light from an LED by optical simulation. As a simplified LED model, a construction formed by superposing a GaN layer measuring the square of 300 μm in area and 6.1 μm in thickness on a sapphire substrate measuring the square of 300 μm in area and 100 μm in thickness was adopted. A point light source capable of isotropically emitting light was disposed at a point forming the center of the square of 300 μm and entering the GaN layer to a depth of 0.1 μm from the GaN surface. The refractive index of sapphire was n=1.8 and that of GaN was n=2.7 (when the wavelength of the emitted light was 380 nm) or n=2.4 (when the wavelength of the emitted light was 400 nm) and the exterior part of the two substances was assumed to be filled with silicone resin having a refractive index of n=1.4. The refractive indexes of GaN at the different wavelengths were determined by subjecting the commercially available GaN bulk substrates to actual measurement. From the point light source, numerous rays of light were emitted in random directions (Monte Carlo method). The rays of light were ramified at the individual interfaces differing in refractivity into the rays of light refracting in accordance with the Fresnel formula and the reflected rays of light at calculated ratios. The number of rays of light emitted was 500,000 and the limit of the cycles of ramification was set at 10. The light collecting surfaces were hypothetically set slightly on resin sides from the interfaces between the resin and each of the back surface of the substrate, the front surface of the semiconductor layer and the lateral surface, and the efficiencies of fetching light from the individual light collecting surfaces were calculated.
Table 1 shows the results of the calculation by simulation of the efficiencies of fetching light from the surface of the substrate, the surface of the semiconductor and the lateral surfaces, respectively in the cases of omitting formation of an irregular construction on the substrate ({circumflex over (1)} and {circumflex over (2)}) and the case of forming an irregular construction shown in FIG. 1 on the surface of the substrate ({circumflex over (3)}.
TABLE 1Lateralsurfaces(GaN lateralsurface andCalculation ofBackFront surfacesapphireefficiency ofRefractivesurface ofof semi-lateralfetching lightindexsubstrateconductorsurface)Total{circle around (1)} LED havingResin7.1%7.4%10.1% × 4 = 40.4%54.9%GaN/sapphiren = 1.4,(7.4%, 2.7%)sealed withGaNsilicone resin,n = 2.4,wavelength ofSapphireemitted light:substrate400 nmn = 1.8{circle around (2)} Same asResin5.1%5.4%7.2% × 4 = 28.8%39.3%above,n = 1.4,(5.4%, 1.8%)wavelength ofGaNemitted light:n = 2.7,382 nmSapphiresubstraten = 1.8{circle around (3)} LED havingResin25.7%5.3%Lateral92.4%irregularn = 1.4,surfaceconstructionGaNperpendicular(FIG. 1)n = 2.7,to grooves incontaining lateralSapphirethe pattern ofsurfaces ofsubstratestripesangle ofn = 1.815.3% (0.5%,inclination of 45°14.8%) × 2,formed inlateral surfacethe interface ofparallel toGaN/sapphire,grooves in thewavelength ofpattern ofemitted light:stripes382 nm15.4% (0.5%,14.9%) × 2
According to the results, when the substrate is not furnished with the irregular construction, the total of efficiencies of fetching light was about 55% when the wavelength of emitted light was 400 nm and about 40% when the wavelength thereof was 382 nm.
These results are applied to the LED disclosed in Journal of Applied Physics mentioned above. This reference, concerning the LED of the semiconductor of the nitride of an element of Group III using a sapphire substrate, has a mention that the external quantum efficiency is 24% when the wavelength of emitted light is 382 nm and 30% when the wavelength thereof is 400 nm. On the assumption that the external quantum efficiency of 24% (24%=internal quantum efficiency of 60%×voltage efficiency of 95%×(efficiency of fetching light of 40%) and the external quantum efficiency of 30% (30%=internal quantum efficiency of 60%×voltage efficiency of 90%×efficiency of fetching light of 55%), the internal quantum efficiency which has no bearing on the wavelength of emitted light can be uniquely indicated as 60% and the results of the simulation seem to be generally appropriate.
According to this simulation, the efficiency of fetching light is about 55% at a wavelength of 400 nm and about 40% at a wavelength of 382 nm, indicating that it has room for improvement to 1.8 times and 2.5 times respectively the ordinary level. Also, the internal quantum efficiency has room for improvement to about 1.6 times the ordinary level. This invention concerns the efficiency of fetching light among other elements involved herein.
A detailed analysis of the results of the simulation has revealed that when the GaN layer and the sapphire substrate are sealed with a resin having a refractive index n=1.4, the ray of light which has permeated the GaN layer and the sapphire substrate is fetched in 100% through the resin to the exterior and that a solution to the question how the group of rays of light entrapped in the GaN layer are fetched to the sapphire substrate and the resin forms an important key to the improvement in the efficient of fetching light.
For the purpose of enabling the rays of light in the GaN layer to permeate the sapphire substrate and the resin, it is only necessary that the interface between the GaN layer and the substrate be inclined and that the angles of incidence of the rays of light on the interface be prevented from exceeding the angle of total reflection. The optimum angle of inclination is 45°. The results of the calculation performed in the case of introducing the construction of irregularities resembling stripes illustrated in FIG. 1 and having an angle of inclination of 45° to the interface between the GaN layer 3 and the sapphire substrate 1 of the case of {circumflex over (3)} given in Table 1 will be shown below. It is noted that the efficiency of fetching light from the sapphire back and lateral surfaces to the exterior is improved, while the efficiency of fetching light from the semiconductor surface through the resin to the exterior is not varied very much. When the wavelength of the emitted light is 382 nm as a total (the refractive index of GaN: 2.7), therefore, it can be expected that the efficiency of fetching light is improved to twice or more the efficiency of the case of {circumflex over (2)}. Incidentally, as regards the ratio of the upper surface, bottom surface and inclined surfaces of the irregular construction, the construction that is destitute of the upper surface and the lower surface and is formed solely of the inclined surfaces proves advantageous because it has the highest efficiency of fetching light.
The present invention has been perfected on the basis of the results of simulation mentioned above.